2010 Group Project 3

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Amniocentesis

Introduction

Developing human embryo showing the surrounding amniotic fluid from which the sample is taken.

Amniocentesis is the process by which a thin needle is inserted through a womens abdomen into her womb, a sample of the amniotic fluid surrounding her growing fetus is taken out and analysed to acquire information about the baby's health.


This procedure is an important development in the technologies of prenatal diagnostic techniques as it allows parents an insight into possible diseases and complications that their baby might develop before birth at an early stage of its development.


Although it is a very useful technique in diagnosing up to several hundred fetal complications there are also many risks associated with this invasive method of extraction of fetal cells as well as ethical issues which will also be explored. A brief history of the development of amniocentesis will be explained, as well as the key disorders which may be detected using it. Lastly we will explore the accuracy of this test against other prenatal diagnostic techniques and highlight current research being conducted.

Historic Background

The removal of amniotic fluid using a needle has been recorded as early as 1930 by Menees, however it was not until 20 years later that the process was actually used in a diagnostic sense. The development of safe and effective techniques of the procedure took many years to evolve, including the necessary technologies to come to par such as ultrasound and chromosome mapping that are key features in ensuring safety and effectiveness in diagnosing the fetus.

This progress could not have been achieved without the work of several key scientists:

Douglas Bevis

Bevis's study of amniocentesis and hemolytic disease was considered a landmark event making the procedure wide spread and launching more interest and research into its possibilities. In February 1952 he published his study in the Lancet, "The Antenatal Prediction of a Hemolytic Disease of the Newborn" which was a revolutionary article at the time. Bevis experimented with the mixing of Rh+ and Rh- blood and what occurs when there is a clash between the mothers Rh factor and her fetus's Rh factor. This results in hemolytic disease, a disease in newborns where the mothers antibodies attack the fetal red blood cells when the antibodies cross the placenta where they are supposed to strengthen the babies immune system. The rhesus factor is present and able to be identified in the fetus from the sixth week of development onwards.[1] The possibility of diagnosing this disorder prenatally was a critical discovery and paved the way for the discovery of hundreds of other disorders which are able to be prenatally diagnosed.

Murray Barr and Ewart Bartram

In the late 1940 's these two scientists began working together studying human sex cells. They discovered in 1949 that the sex of a fetus could be found by identifying an extra chromatin body which lies at the periphery of only female cells, named the Barr body, which is visible under the microscope. The significance of this in relation to amniocentesis as a diagnostic technique is that the possibility of sex linked diseases such as hemophilia, a blood clotting disorder, could be found out prenatally. Until this discovery parents were given statistics based on their family histories of sex linked diseases which were nothing more than loose probability. With the sex of the baby able to be determined more reliable statistics could be calculated.

Fuchs and Riis

Dr. Carlo Valenti

Procedure

Who is eligible for the test?

Since the procedure is invasive and not without a number of risks, the mother is tested before to determine whether she is at a higher risk than average of having a child with a chromosomal or neural tube defect. These tests include:

  • Checking for abnormal ultrasound features.
  • Maternal age is considered, women older than 35 are at a higher risk of having children with chromosomal disorders.[2]
  • History of chromosomal disorders in the family, testing if the parents are Rh positive or Rh negative.[3]
  • History of previous children born with a genetic defect.


When can the test be taken?

An amniocentesis test is most commonly performed between weeks 15 to 16 of gestation which is during the second trimester of pregnancy, however it is able to be performed anytime between weeks 14 to 20. Amniocentesis during the third trimester is strongly avoided as there is a high risk of inducing an early labor from premature rupture of membranes within the womb.[4]. Second trimester procedures are preferred over an early amniocentesis which is performed in the first trimester during weeks 11 to 14 as the earlier the test is performed the higher the risk of miscarriage, as much as 3 times greater. The reason an early amniocentesis option is offered to women is since the delay in tissue analysis of about 2 weeks means if an abortion is required it is physically and emotionally easier earlier on in the pregnancy. If an early diagnostic test is requested Chorionic Villus Sampling (CVS) is suggested as it carries less risk of miscarriage and complications than amniocentesis in the first trimester.[5]


Steps of the procedure

The amniocentesis procedure showing insertion of the needle into the uterus and use of ultrasound.
  1. Counseling for the mother is provided to investigate any family history of genetic disorders or any birth defects in previous children. The mother is given an assessment[6] on whether this procedure will benefit her based on her history and likelihood of baring a child with a fetal abnormality. She is also informed of the details and risks of the procedure and is able to make an informed decision to continue or not based on anxiety towards an invasive and risky procedure. [7][8]
  2. The next step is cleaning of the area before needle insertion with an iodine solution to ensure no contamination.
  3. Local anesthetic may be administered but since there is little pain it is usually omitted to avoid a second needle insertion.
  4. The needle is inserted and 15 mL of amniotic fluid is sampled, it takes approximately 30 seconds to withdraw the fluid.
  5. Before the needle is inserted and whilst the needle is inside the uterus ultrasound is used continuously to ensure no harm is caused to the baby or placenta.
  6. After the fluid is sampled the baby's heart beat is checked to ensure there was no harm.
  7. The fluid containing DNA of the fetus from shed skin cells during development is sent to a lab where it is cultured and the chromosomes are able to be mapped to investigate any chromosomal defects. Protein levels are also analyzed to investigate any neural tube defects.[9]
  8. In the lab the amniotic fluid sample is centrifuged to separate the fetal cells from the amniotic fluid containing lipids, proteins, glucose and electrolytes.
  9. After it is centrifuged the bottom half of the separated fluid containing the fetal cells is immersed in a culture medium designed to maximize colony growth of the embryonic cells. The cells are able to grow and then stored in an incubator until the time comes to harvest them.
  10. The harvested cells are arrested in the metaphase stage of cell division which is the time of most condensation of the DNA. This means the chromosomes are able to be seen under a microscope after correctly dyed.
  11. The chromosomes are stained with certain chemicals that bind to the DNA to create a characteristic and recognizable banding pattern that will be different for the varying molecular structures of the chromosomes.
  12. The standard way of observing the stained chromosomes is by creating a karyotype. A karyotype is a photograph taken of the chromosomes arranged from largest to smallest non-sex chromosomes (autosomal) numbered 1 to 22, with the 2 sex chromosomes placed at the end. This allows analysis to be done efficiently and any structural abnormalities to be clearly identified.
  13. Structural abnormalities that may be indicative of a genetic disorder include chromosome deletions, duplications, translocations and inversions.

Risks


Disorders Detected

Chromosomal mapping can highlight chromosomal defects such as Down syndrome occurring due to an extra copy of chromosome 21, trisomy 21.


Diagnostic Accuracy

Ethical Issues

Current Research

References

  1. <pubmed>4255486</pubmed>
  2. <pubmed>6220164</pubmed>
  3. <pubmed>15832534</pubmed>
  4. <pubmed>18191913</pubmed>
  5. Prenatal Testing: An Amniocentesis Primer. (2004). http://www.plus-size-pregnancy.org/Prenatal%20Testing/prenataltest-amnios.htm
  6. <pubmed>9084385</pubmed>
  7. Brajenović-Milić B, Babić I, Ristić S, Vraneković J, Brumini G, Kapović M. Womens Health Issues (2008) Pregnant women's attitudes toward amniocentesis before receiving Down syndrome screening results.PMID: 18180167
  8. <pubmed>15284930</pubmed>
  9. North East Valley Division General Practice Victoria, Australia.http://www.nevdgp.org.au/info/melb_us/Amniocentesis_melb.htm

2010 ANAT2341 Group Projects

Project 1 - Ultrasound | Project 2 - Chorionic villus sampling | Project 3 - Amniocentesis | Group Project 4 - Percutaneous Umbilical Cord Blood Sampling | Project 5 - Fetal Fibronectin | Project 6 - Maternal serum alpha-fetoprotein | Group Assessment Criteria

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Cite this page: Hill, M.A. (2024, March 29) Embryology 2010 Group Project 3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2010_Group_Project_3

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G